Compatible, non-crosslinked thermoplastic materials can usually be readily connected. Well-known techniques for this include joining respective sections by using adhesives and welding procedures. Pipe ends of crosslinked polymer, however, particularly of crosslinked polyolefins, are not readily joined by available techniques to each other or to pipe ends of non-crosslinked polymers. It would be desirable to have a process available for connecting pipe fittings made from crosslinked polyolefins with similar pipes or pipes made from non-crosslinked polyolefins.